A printed circuit board or PCB is a carrier used for the electrical connection of a set of electrical components. Such a printed circuit board generally takes the form of a stratified or laminated plate. This printed circuit board may be a single-layer or multilayer printed circuit board. A single-layer printed circuit board has only one metallization layer in which there are printed conductive tracks that electrically connect the different electrical components to one another. A multilayer printed circuit board on the contrary has several metallization layers, i.e. at least two layers and, preferably, more than four or six layers. The description here below shall be concerned chiefly with these multilayer printed circuit boards.
A metallization layer is one of the layers of the stratified plate forming the printed circuit board in which one or more conductive tracks are made, electrically connecting the different electrical components to one another. This layer is flat and extends in parallel to the plane of the stratified plate. Generally, the metallization layer is obtained by depositing a uniform layer of a conductive material, typically a metal such as copper, and then etching this uniform layer to allow only the conductive tracks to remain.
The different metallization layers of the printed circuit board are mechanically spaced out from one another by insulating layers made of an electrically insulating material. This insulating material has high dielectric rigidity, i.e. typically greater than 3 MV/m and preferably greater than 10 MV/m. For example, the electrically insulating material is made of epoxy resin and/or fiberglass. The insulating layer generally takes the form of a rigid plate made of a material that does not become viscous when it is joined with other layers. For example, it is made of a thermosetting resin which has already undergone irreversible thermosetting.
The different layers of the multilayer printed circuit board are joined to one another without any degree of freedom by means of adhesive layers known as “pre-impregnated” layers and more generally known as “prepreg” layers.
A pre-impregnated layer is constituted by an thermosetting resin impregnating, generally, a reinforcing element such as a fabric. Typically, the resin is an epoxy resin. During the manufacturing of the printed circuit board, the transformation of the thermosetting resin brings into play an irreversible polymerization which converts the pre-impregnated material into a solid and rigid material which irreversibly bonds together the different layers of the printed circuit board. Typically, each transformation takes place when the pre-impregnated layer is heated to a high temperature and is compressed with high pressure. Here, a high temperature is a temperature above 100° C. and preferably above 150° C. A high pressure is a pressure greater than 0.3 MPa and typically greater than 1 MPa.
The conductive tracks of the different metallization layers can be electrically connected by means of conductive pads passing through the insulating layers. The conductive pads are more generally known as “vias” or “via holes”. The vias generally extend perpendicularly to the plane of the layers. There are different ways of making these vias. One of the most common ways is to make a hole in the insulating layer or layers to be crossed and then to coat the inner wall of these holes with a metal. They are then referred to as metallized holes.
A via does not necessarily pass through all the layers of the printed circuit board. Thus, there are blind holes which open into only one external face of the printed circuit board. At present, it is also possible to make “buried” vias for example by means of known technologies such as HDI (High Density of Integration) technologies. A buried via does not open into any of the external faces of the printed circuit board. For example, a buried via electrically connects conductive tracks made in metallization layers buried within the printed circuit board.
Prior-art printed circuit boards comprise: a stack, along a vertical direction, of several metallization layers separated mechanically from one another by electrically insulating layers, at least one coil that extends along a vertical winding axis, the turns of this coil being formed by conductive tracks made in respective metallization layers, electrically connected to one another by pads going through at least one of the insulating layers.
Such printed circuits are for example known from U.S. Pat. No. 7,511,591, EP 1168387, and U.S. Pat. No. 4,080,585.
In the prior-art printed circuit boards, it has been noted that the magnetic field generated by the coil along its winding axis has little homogeneity and has a bias as compared with the magnetic field which would be generated by an ideal coil. This problem of inhomogeneity of the magnetic field thus falsifies the measurement of such a coil.